When I looked at this a year ago I found that SSAS 2008 R2 doesn't play well with Vertica v6 in ROLAP mode.

Here's the use case you will want to test, assuming you're using a more recent version of SSAS and Vertica.

Create a Vertica table called CustomerTransaction with three columns (CustomerID, TransactionID, TransactionDate). Populate it with data.
Create a SSAS cube in ROLAP mode. Create a metric in SSAS that counts the number of Unique Customers that had a TransactionDate between a Start Date and an End Date. Look at the SQL that SSAS generates in the vertica system table v_monitor.query_requests. Ideally, the BI tool would generate a SQL statement such as: